The present invention relates generally to amplifiers, and more particularly to a precision bridge amplifier having two single-ended amplifiers.
Amplifiers can be employed to amplify electronic signals of small magnitudes to produce corresponding electronic signals having larger magnitudes. Amplifiers are accordingly used in electronic systems in a variety of applications where it is beneficial to control large loads with small signals. A particular type of amplifier configuration is a bridge amplifier. A typical bridge amplifier includes two single-ended power amplifiers that are driven out of phase. By connecting a load across the outputs of the two amplifiers, the available peak voltage to the load is double that which would otherwise be available from one single-ended amplifier. Bridge amplifiers are useful in applications where the supply voltage is limited and in applications where single-ended amplifiers are operating near their maximum voltage ratings.
Conventional bridge amplifiers utilize a master-slave configuration, wherein one of the single-ended power amplifiers functions as a master amplifier and has a gain of a given magnitude to provide an amplified control signal at its output. The second of the single-ended power amplifies functions as a slave amplifier having a unity gain and simply inverts the output of the master amplifier. Using this approach, ideally the available peak voltage differential across the outputs of the master-slave amplifiers will be twice that available solely from the master amplifier. In practical applications, however, distortion/infidelity can easily be created in the output waveform throughout the frequency range, as a result of the feedback control loops for the master and slave amplifiers sensing only their associated single-ended output voltage and the gain/offset mismatch between the two amplifiers. Distortions are not desirable in applications where the associated load is highly voltage sensitive and demands a high level of control over the voltage waveform.
In view of the above, there is a desire for a bridge amplifier that reduces distortion/infidelity in the output waveform.
One aspect of the present invention provides a precision bridge amplifier including a first input node connectable to a power source having an input voltage, a second input node connectable to a control source having a control voltage, and a first and second output node. A first amplifier module is coupled between the first and second input nodes and between the first and second output nodes, and has a gain. A second amplifier module is coupled to the first input node and between the first and second output nodes. The first amplifier module compares a voltage differential between the first and second output nodes to the control voltage, and provides an output voltage at the first output node necessary to maintain the voltage differential at a level substantially equal to a product of the control voltage multiplied by the gain. The second amplifier module determines a midpoint voltage level of the input voltage and a midpoint voltage level of the voltage differential, and provides an output voltage at the second output node necessary to maintain the voltage differential midpoint level at a level substantially equal to the input voltage midpoint level.
In one embodiment, the first amplifier module includes an error amplifier circuit coupled to the second input node. A power amplifier circuit is coupled to the first input node, to a ground node, and between the error amplifier circuit and the first output node. A feedback circuit is coupled to the error amplifier circuit, to the ground node, and between the first and second output nodes.
In one embodiment, the error amplifier circuit of the first module comprises an operational amplifier having a non-inverting terminal coupled to the second input node, an inverting terminal, a positive voltage terminal, a negative voltage terminal, and an output terminal coupled to the power amplifier circuit. A first resistor is coupled between the inverting terminal and the feedback circuit. A second resistor has a first terminal coupled to the inverting terminal and a second terminal. A capacitor is coupled between the output terminal and the second terminal of the second transistor.
In one embodiment, the power amplifier circuit of the first module comprises an inverting power amplifier having an input terminal coupled to the error amplifier circuit output terminal, a positive voltage terminal coupled to the first input node, a negative voltage terminal coupled to the ground node, and an output terminal coupled to the first output node.
In one embodiment, the feedback circuit of the first module comprises an operational amplifier having a non-inverting terminal, an inverting terminal, a positive voltage terminal, a negative voltage terminal, and an output terminal coupled to the error amplifier circuit. A first resistor is coupled between the inverting terminal and the first output node, and a second resistor is coupled in parallel with the first resistor. A third resistor is coupled between the non-inverting terminal and the second output node, and a fourth resistor is coupled in parallel with the third resistor. A fifth resistor is coupled between the inverting terminal and the output terminal, and sixth resistor is coupled between the non-inverting terminal and the ground node. In one embodiment, the first, second, third and fourth resistors have substantially equal values, and the fifth and sixth resistors have substantially equal values.
In one embodiment, the second amplifier module includes an error amplifier circuit coupled to a ground node. A power amplifier circuit is coupled to the first input node, to the ground node, and between the error amplifier circuit and the second output node. A feedback circuit is coupled to the first input node, the error amplifier circuit, to the ground node, and between the first and second output nodes.
In one embodiment, the error amplifier circuit of the second module includes an operational amplifier having a non-inverting terminal coupled to the ground node, an inverting terminal, a positive voltage terminal, a negative voltage terminal, and an output terminal coupled to the power amplifier circuit. A first resistor is coupled between the inverting terminal and the feedback circuit. A second resistor has a first terminal coupled to the inverting terminal and has a second terminal. A capacitor is coupled between the output terminal and the second terminal of the second resistor.
In one embodiment, the power amplifier circuit of the second module comprises and inverting power amplifier having an input terminal coupled to the error amplifier circuit output terminal, a positive voltage terminal coupled to the first input node, a negative voltage terminal coupled to the ground node, and an output terminal coupled to the second output node.
In one embodiment, the feedback circuit of the second module includes an operational amplifier having a non-inverting terminal, an inverting terminal, a positive voltage terminal, a negative voltage terminal, and an output terminal coupled to the error amplifier circuit. A first resistor is coupled between the inverting terminal and the second output node. A second resistor is coupled between the inverting terminal and the first output node, and has a value equal to the first resistor. A third resistor is coupled between the non-inverting terminal and the first input node. A fourth resistor is coupled between the non-inverting terminal and the ground node, and has a value substantially equal to the third resistor. A fifth resistor is coupled between the inverting terminal and the output terminal, and a sixth resistor is coupled between the non-inverting terminal and the ground node. In one embodiment, the first, second, third and fourth resistors have substantially equal values, and the fifth and sixth resistors have substantially equal values.
In one embodiment, the first and second amplifier modules have passive components having substantially equal values. In one embodiment, the first and second amplifier modules having active components having substantially equal values. In one embodiment, the first and second amplifier modules have both active and passive components having substantially equal values.
In one embodiment, the first amplifier module is substantially identical to the second amplifier module. In one embodiment, the first and second amplifier modules each include external input and external output terminals, wherein the external input and external output terminals of the first amplifier module are coupled into the bridge amplifier differently than the external input and external output terminals of the second amplifier module. In one embodiment, the first and second amplifier modules are each encapsulated in plastic with access only to the external input and external output terminals.
One aspect the present invention provides a method of providing a desired voltage differential between a first node and a second node. The method includes receiving an input voltage with a midpoint voltage level from a power source and a control voltage from a control source. The method includes providing a voltage differential between the first node and the second node. The voltage differential between the first and second nodes is compared to the control voltage, and a voltage level necessary to maintain the voltage differential at a level substantially equal to the desired voltage level is provided at the first node. The desired voltage level is substantially equal to the control voltage multiplied by a gain. A midpoint level of the voltage differential between the first and second nodes is compared to the power source midpoint voltage level and a voltage level necessary to maintain the midpoint voltage level of the voltage differential at a level substantially equal to the power source midpoint voltage level is provided at the second node.